1. Field of the Invention
The present invention relates to a facile synthetic method of a series of 2-(4′-alkoxyphenyl)-5-cyanopyridine liquid crystal compounds, and more particularly to a synthetic method which uses the Grignard reagent and promotes electrophilic properties of pyridines in the synthesis.
2. Description of the Prior Art
With the rapid development of internet, communication and computer manufacturing technologies, a variety of portable electronic products such as cellular phones, mobile computers, personal digital assistants (PDA), palm audio/visual players (e.g. MP3 and MP4 players), and the like, are playing an indispensable role in modern life. As the present electronics moves in a trend of thinner products, liquid crystal displays have been gradually taking the place of conventional cathode ray tubes. Because of their advantages in thin structure, light weight, low power consumption, low radiation contamination, and compatibility with semiconductor processing technology, liquid crystal displays have found broad applications in diverse electronic products. In order to advance liquid crystal manufacturing technology, it is essential to further improve and develop liquid crystal materials.
Because of its advantages in high response speed, high resolution, and wide viewing angle, the Highly Sensitive Electric Field Induced Liquid Crystal has become a common optical switching material used in the new generation of liquid crystal displays. Among the Highly Sensitive Electric Field Induced Liquid Crystals, 2-(4′-alkoxyphenyl)-5-cyanopyridine liquid crystal compounds are similar in molecular structure to heptyloxycyanobiphenyl (7OCB), which has been widely used in commerce. Therefore, the former has a great potential to be used as molecular optical switches in liquid crystal displays.
Conventional synthesis of 2-(4′-alkoxyphenyl)-5-cyanopyridine was disclosed by Pavelyuchenko, A. I. et al. in 1986 (Pavelyuchenko, A. I., T. A. Mikhailova, E. I. Kovshev, V. V. Titov, Nauchno-lssled, Inst. Zhurmal Organicheskoi Khimmi, 1986, 22(5): 1061-5). As shown in FIG. 1, the conventional synthesis of 2-(4′-alkoxyphenyl)-5-cyanopyridine includes the following steps:                1. Sodium benzoylacetaldehydide (14) was obtained by the condensation of alkoxyacetophenones (11) with ethyl formate (HCO2C2H5) (12) and sodium (13) in ether and gave yields of 70-76%.        2. Arylcyanopyridine (16) was obtained by the cyclization of sodium benzoylacetaldehydide (14) with cyanoacetamide (CNCH2CONH2)(15) in a water-dioxane mixture. The yield of the arylcyanopyridine (16) amounted to 60-65% with a water-dioxane ratio of 1:10.        3. The substitution of the oxygen of the carbonyl group by a chlorine atom in arylcyanopyridine (16) was taken place on heating with phenylphosphonyl chloride (PhPOCl2)(17), and the yields of 6-(4′-alkoxyphenyl)-2-chloro-3-cyanopyridines (18) amounted to 70-80%.        4. The chlorine of the carbonyl group in 6-(4′-alkoxyphenyl)-2-chloro-3-cyanopyridines (18) were reduced by catalytic hydrogenation over 10% Palladium/Carbon (Pd/C)(19), and the yields of the obtained 2-(4′-alkoxyphenyl)-5-cyanopyridine (I) amounted to 39-60%. Then, the synthesis of 2-(4′-alkoxyphenyl)-5-cyanopyridine (I) was completed after the purification of the obtained 2-(4′-alkoxyphenyl)-5-cyanopyridine (I).        
The four-step synthetic method mentioned above involves many procedures and consumes a large amount of reagents; moreover, some of the reagents are costly, especially the rare element palladium used in the fourth step. Worst of all, the total yield is only 8.5-13.4%. For these reasons, the method is not suitable for industrial production.
Accordingly, there are still many disadvantages in the conventional synthesis of 2-(4′-alkoxyphenyl)-5-cyanopyridine liquid crystal compounds, and it needs to be improved.
The inventor, in view of the drawbacks of the conventional synthetic method of 2-(4′-alkoxyphenyl)-5-cyanopyridine liquid crystal compounds, after many years of R&D on innovative improvement, has successfully developed a facile synthetic method of a series of 2-(4′-alkoxyphenyl)-5-cyanopyridine liquid crystal compounds.